In general, in a conventional image forming apparatus, a sheet detecting apparatus configured to detect a position of a leading edge of a sheet is provided on a sheet conveying path so as to synchronize a timing to send the sheet to an image transfer position with a timing to send an image formed by an image forming portion to the image transfer position (PTL 1). The sheet detecting apparatus is also usable for detecting a sheet conveying status on the sheet conveying path, such as a sheet conveyance delay and a jam.
FIG. 15 illustrates a conventional sheet detecting apparatus 100. As illustrated in FIG. 15, the conventional sheet detecting apparatus 100 is provided on a downstream side in a sheet conveying direction (hereinafter simply referred to as “downstream side”) with respect to a pair of conveying rollers 131 and 132, which is located on an upstream side in the sheet conveying direction (hereinafter simply referred to as “upstream side”) so as to be closest to the image transfer position. The sheet detecting apparatus 100 includes a lever member 173 which abuts against a sheet S, an optical sensor 175b, a light blocking flag 175a configured to block an optical path from a light emitting portion to a light receiving portion of the optical sensor 175b, and a stopper 176 configured to position the lever member 173 in a waiting position. The lever member 173 is configured to be rotatable about a rotary shaft 173c, and to return, after the rotation, to the waiting position by a pressing force of a return spring 174. The light blocking flag 175a is formed integrally with the lever member 173, and rotates together with the lever member 173.
As illustrated in FIGS. 16A and 16B, when a leading edge of the sheet S abuts against the lever member 173, the lever member 173 rotates about the rotary shaft 173c from a home position HP in a direction indicated by the arrow in FIG. 16B, and the light blocking flag 175a blocks the optical path of the optical sensor 175b. When the optical sensor 175b detects that the optical path is blocked, the sheet detecting apparatus 100 recognizes that the leading edge of the sheet S reaches the lever member 173. After that, the sheet S pushes the lever member 173, and the lever member 173 rotates accordingly, with the result that the sheet S is allowed to move. When a trailing edge of the sheet S is separated from the lever member 173, the lever member 173 is rotated by the return spring 174 in a direction opposite to the direction indicated by the arrow in FIG. 16B, to thereby return to the home position HP. At this time, the light blocking flag 175a retracts from the optical path, and the light receiving portion of the optical sensor 175b again receives the light emitted from the light emitting portion thereof, with the result that the sheet detecting apparatus 100 recognizes that the trailing edge of the sheet S has passed the lever member 173.
By the way, in recent years, much higher throughput (number of sheets subjected to image formation per unit time) of an image forming apparatus has been demanded. In order to meet this demand, it has been required to convey sheets at a higher speed, and reduce a distance between a trailing edge of a preceding sheet and a leading edge of a succeeding sheet (hereinafter referred to as “sheet-to-sheet distance”). In accordance therewith, it is necessary for the lever member to be returned to the home position HP within a short sheet-to-sheet distance.
On the other hand, when the leading edge of the sheet S which has passed through the pair of conveying rollers 131 and 132 abuts against an abutment surface 173a of the conventional lever member 173, the lever member 173 is pushed by the sheet S to rotate, and when the trailing edge of the sheet S is separated from the abutment surface 173a, the lever member 173 reversely rotates to return to the home position HP. Therefore, a distance required as the sheet-to-sheet distance is a total distance of a distance D1 from a position in which the trailing edge of the preceding sheet has passed the abutment surface 173a of the lever member 173 to the home position HP in which the leading edge of the succeeding sheet abuts against the abutment surface 173a, and a distance D2 required for conveying the succeeding sheet to the home position HP while the lever member 173 is returned to the home position HP (FIG. 16C). The lever member 173 performs reciprocating rotation, and hence the distance D1 is generated so as to return the lever member 173 to the home position HP after the sheet S passes the lever member 173, and the lever member 173 takes a time ΔT for moving the distance D1. On the other hand, the distance D2 is a distance (ΔT×V) obtained by multiplying the time ΔT during which the lever member 173 moves the distance D1 by a conveying speed V of the sheet S. As the conveying speed V of the sheet S becomes higher, the distance becomes longer. Therefore, in the conventional sheet detecting apparatus 100, when the conveying speed V of the sheet S is increased, the sheet-to-sheet distance needs to be set longer, and hence further enhancement of the throughput is practically impossible. Thus, in the apparatus configured to detect the sheet by using the lever, enhancement of throughput of the sheet conveyance has been limited due to a time period for returning the lever.